Monkey kidney cells (Vero) growth in tissue culture in 35 mm Petri dishes have been observed in a calorimeter possessing long term stability of 0.5 microwatts. Cells in monolayer culture in maintainance medium were found to give a constant level of heat production that was stable for over five days. This base line characteristic of living cells was established to be exothermic relative to the identical instrumental base line exhibited by media alone and cells killed by exposure to either methanol, formaldehyde of Gamma-radiation. Infection of living Vero cells by an attenuated strain of poliovirus exhibited a broad inverse sigmoidal transition between these two regimes. Microscopic examination showed the fibroblasts to become progressively rounded-up and detached concurrent with the decrease in heat production. The characteristic time parameters of the decreased heat production were found to be inversely proportional to the viral inoculum over a 10 to the seven-fold range of initial virus concentration, demonstrating that calorimetry may be used for the assay of viral titre. Inadverant bacterial contamination of the sample is rapidly detected by calorimetry.